Update documentation to include Windows Magefile Cache (#484)

* Update _index.en.md

* Update documentation to include Windows Magefile Cache

Author: WilliamRoyNelson

site/content/environment/_index.en.md

@@ -14,7 +14,7 @@ Set to "1" or "true" to turn on debug mode (like running with -debug)
 ## MAGEFILE_CACHE
 
 Sets the directory where mage will store binaries compiled from magefiles
-(default is $HOME/.magefile)
+(default is $HOME/. or %USERPROFILE%\magefile)
 
 ## MAGEFILE_GOCMD
 

site/content/howitworks/_index.en.md

@@ -14,12 +14,13 @@ binary is also used in the hash.
 
 ## Binary Cache
 
-Compiled magefile binaries are stored in $HOME/.magefile.  This location can be
-customized by setting the MAGEFILE_CACHE environment variable.
+Compiled magefile binaries are stored by default in $HOME/.magefile or in
+%USERPROFILE%\magefile.  This location can be customized by setting the
+MAGEFILE_CACHE environment variable.
 
 ## Go Environment
 
 Mage itself requires no dependencies to run. However, because it is compiling go
 code, you must have a valid go environment set up on your machine.  Mage is
 compatible with any go 1.7+ environment (earlier versions may work but are not
-tested).
+tested).